The present invention relates to a method of controlling the flow rate of air intake of an internal combustion engine, and to apparatus for carrying out the method. The invention particularly relates to an air intake system and method for controlling a throttle valve at its fully closed position.
There is a known method of controlling the air intake of an internal combustion engine when a throttle valve in an intake passage is fully closed. According to this conventional method, the flow rate of intake air when the throttle valve is fully closed is controlled by adjusting the cross-sectional area of a bypass passage around part of a flow passage. The cross-sectional area is adjusted by means of a control valve in the air bypass passage which connects a first part of the intake passage at a location upstream of the throttle valve with a second part of the intake passage at a location downstream of the throttle valve. Such an air intake control method is usually employed for controlling the idling rotational speed of the engine. The idling rotational speed can be controlled if the control valve is adjusted to control the flow rate of the air that passes into the engine through the bypass passage so that the detected actual rotational speed of the engine becomes equal to the desired idling rotational speed.
Among the systems for controlling the flow rate of the intake air, some are equipped with a deceleration control function that causes the valve for controlling the cross-sectional area of the flow passage to increase its opening when the throttle valve is changed from an opened position to a closed position, in order to prevent the flow rate of the intake air from suddenly decreasing when the engine starts to decelerate. The purpose of the deceleration control function is to prevent the mixture gas from becoming too rich when the engine is decelerating and to maintain a suitable flow rate of the intake air to improve combustion and to prevent misfiring in order to restrain excessive emission of carbon monoxide and unburned hydrocarbons. In an engine without an air bypass passage, the deceleration control function is achieved by, for example, a throttle return check mechanism or dashpot which prevents the throttle valve from being quickly closed when the foot of the driver is separated from the accelerator pedal.
When the deceleration control function is added to a system for controlling the air intake according to the conventional art, a throttle closure switch detects whether the throttle valve is fully closed or not, and the control valve in the bypass intake passage is actuated toward the opening direction, depending solely upon the detected result from the throttle closure switch. When the throttle valve approaches the fully closed position, however, the throttle closure switch cannot clearly discriminate whether it is fully closed or not, and the throttle closure switch switches repeatedly between its turn-on and turn-off operations. Consequently, under light load conditions in which the throttle valve is close to the fully closed position, the control valve in the bypass intake passage undergoes the opening and closing operations. Therefore, the flow rate of the intake air undergoes variation, which makes it difficult to continue the operation in a stable manner under light load conditions.